1. Field of the Invention
The present invention relates to Chlamydia psittaci vaccines and to methods of protecting animals, including avian species, from Chlamyida psittaci infections.
2. Background Art
The genus Chlamydia contains four species of obligate parasitic bacteria: Chlamydia psittaci, Chlamydia pecorum, Chlamydia pneumoniae, and Chlamydia trachomatis. This unique genus causes a variety of diseases in humans, mammals, and birds. In humans, the most notable are trachoma and urogenital infections due to C. trachomatis and psittacosis caused by C. psittaci. In animals, C. psittaci can cause a diverse range of disease in livestock, poultry, turkeys and companion birds. The known C. psittaci strains have been grouped into eight biovars (Perez-Martinez, J A and J Storz, 1985). Strains of serovar 1 are mainly associated with intestinal infections and abortions, while strains of serovar 2 cause polyarthritis, encephalitis, and conjunctivitis in ruminants. Avian strains of C. psittaci cause respiratory problems and diarrhea in birds (Storz, 1988). The organism can also be transmitted to humans from these animals, and outbreaks have been documented in animal production workers. Thus, there is a need for an effective vaccine against C. psittaci for mammalian and avian species.
The chlamydia organism goes through two developmental stages in its life cycle. The extracellular form, which is the infectious entity of the cycle, is called the elementary body (EB). These EBs attach and enter the host cell, where they re-organize into reticulate bodies (RBs) which divide within membrane-bound host cell compartments by binary fission and then condense into a new generation of infectious EBs. The attachment and entry of the EB into the host cell is a receptor-mediated phenomenon (Hodinka et al. 1988), and several chlamydial proteins have been implicated in the EB attachment to host cellular membranes (Baghian and Schnorr, 1992). One of these proteins is called the “major outer membrane protein”, or MOMP, and surface-exposed epitopes of this protein from C. trachomatis have been shown to block EB attachment onto the host cell (Su and Caldwell, 1991). The MOMP genes from some strains of C. psittaci and C. trachomatis have been sequenced (Baehr et al., 1988, Pickett et al. 1988, Yuan et al. 1989, Zhang et al. 1989, Kaltenboeck, et al. 1993). Analyses of these sequences revealed that portions of the structure of this protein are conserved between species. There are also four regions of “variable domain” interspersed with conserved sequences, and these are referred to as VD1, VD2, VD3, and VD4. The location of these VD1 regions are identical in the two species (see Zhang et al., 1989). A comparison of the genes encoding the MOMP from C. psittaci and C. trachomatis show that, overall, the sequences are approximately 68% identical.
In C. trachomatis, these four variable regions have been shown to be involved in the neutralization of EB infectivity, in serotype specificity, (Baehr, et al. 1988; Peeling et al. 1984; and Spears and Storz 1979) as well as in the pathogenicity of the strains (Baehr et al. 1988 and Su et al. 1988). Nonetheless, the development of subunit vaccines for C. trachomatis has been hampered by the difficulty in expressing the native, full-length MOMP gene in a recombinant vector host (Manning and Stewart, 1993). There is no known published work on the expression of the C. psittaci MOMP gene prior to that described herein. Consequently, there remains a need to develop an effective subunit vaccine for animal and avian species to protect them from C. psittaci infections.